The hunt for neutrinos in the Antarctic

The IceCube project has constructed a giant detector in the Antarctic ice to find subatomic particles, which could reveal where cosmic rays come from — and their cause.

By Ian Sample / The Observer, LONDON

Illustration: Yusha

Spencer Klein is holding a thick glass ball the size of a watermelon and it is stuffed with electronics. For 10 minutes or so, he turns it over in his hands and talks through what it does, how it works and the brutal environment it can withstand. This last point turns out to be key. Over the past half-decade, more than 5,000 of these things have been shipped to the South Pole, strung together like beads and buried deep in the Antarctic ice sheet.

Klein is a physicist at the Lawrence Berkeley National Laboratory that sits high on the hills overlooking the University of California’s Berkeley campus. The glass ball in his hands is a “digital optical module” (DOM), an exquisitely sensitive light detector that lies at the heart of what must be one of the most ambitious projects in the history of science. By freezing these modules into the ground around the US Amundsen-Scott South Pole station, Klein and his colleagues have turned a cubic kilometer of pristine polar ice into an enormous cosmic observatory.

The US$272 million IceCube instrument is not your typical telescope. Instead of collecting light from the stars, planets or other celestial objects, IceCube looks for ghostly particles called neutrinos that hurtle across space with high-energy cosmic rays. If all goes to plan, the observatory will reveal where these mysterious rays come from and how they get to be so energetic. Although that is just the start. Neutrino observatories such as IceCube will ultimately give astronomers fresh eyes with which to study the universe.

The frigid conditions at the South Pole meant construction teams could only work on IceCube between November and February each year when ski-equipped planes can safely make the 2,900km round trip to the research station. The DOMs are designed to run on precious little power, a measly 5W each, but even so, it takes 10 planeloads of fuel to run IceCube for a single year.

The final piece of the observatory was put in place a week before Christmas when engineers used a hot-water drill to melt the last of 86 holes in the ice. The holes reach a depth of 2.5km and down each is lowered a string of 60 DOMs that are locked in place when the water in the hole refreezes. The pressure is so great at these depths that air bubbles are squeezed out of the ice, leaving it almost perfectly transparent.

Physicists on the IceCube project are now completing a series of checks on the latest additions to their bizarre instrument to see if the equipment survived being installed. Assuming it has — and only a couple of DOMs have failed in the project’s history — the instrument will soon swing into action and its search for cosmic rays will begin in earnest.

“Our best calculations show that we need an instrument this size to have a good chance of seeing these cosmic ray sources,” Klein says. “Now we’re done, we have it.”

An Austrian-born scientist called Victor Hess discovered cosmic rays 100 years ago. In a series of hot-air balloon flights, Hess measured the radiation around him at altitudes up to and beyond 5km. As he rose up through the atmosphere, radiation levels initially fell, but then rose steeply until they were double that at sea level. Hess reasoned that radiation must somehow reach Earth from outer space.

Cosmic rays are now known to be highly energetic particles that originate in outer space and bombard our planet from all directions. Most are made up of charged particles, such as metal ions, but these are of little use to space scientists hoping to discover the origins of high-energy cosmic rays. Charged particles are deflected by magnetic fields as they race across space, making it hard, or impossible, to retrace their route and locate their cosmic birthplace.